1. Field of the Invention
The present invention relates to a method of controlling a guard interval section length in an orthogonal frequency division multiplexing (OFDM) method and the OFDM transmitting and receiving apparatuses which controls a guard interval section length to a line quality adaptively.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) method is known as a radio transmission method which has excellent anti-multi-path characteristic.
FIG. 1 is a diagram showing the principle of a signal transmission method in a conventional OFDM transmitting and receiving apparatus. As shown in FIG. 1, in the OFDM method, a guard interval section is provided between effective symbols by the assumed maximum delay time of a transmission signal transmitted from a transmitting apparatus, in order to exclude the influence of interference between the transmission signal and a multi-path signal. A part of the following effective symbol is copied and added to the guard interval section, and then the transmission signal is transmitted. In a receiving apparatus, since the guard interval section which degrades due to the interference between symbols, the guard interval section is removed. Thus, the interference between symbols in the multi-path is restrained.
In the signal transmission system of such an OFDM system, when the guard interval section length is longer, it is possible to cope with the larger delay in the multi-path. However, the data transmission efficiency degrades if the guard interval section is made longer than a necessary length. Therefore, excellent anti-multi-path characteristic can be realized, if the guard interval section length is made minimum and the guard interval section length is adaptively controlled in accordance with the change of a propagation environment when the maximum delay of the delayed signal changes due to the change of the propagation environment. Also, it is possible to avoid the reduction of the data transmission efficiency.
As a conventional method of adaptively controlling the guard interval section length is known in Japanese Laid Open Patent application (JP-P2000-244441A), in which a signal is multiplexed to one of sub-carriers to estimate an optimal guard interval section length. Hereinafter, the structure of the OFDM transmitting and receiving apparatus in this conventional example will be described.
FIGS. 2A and 2B are diagrams showing the OFDM transmitting and receiving apparatuses, in which data of a guard interval section length is multiplexed with one of sub-carriers and transmitted. The transmitting apparatus 201 is composed of a serial/parallel converting section 103, a first switch 203, an inverse Fourier transforming section 104, and a guard interval adding section 105. The serial/parallel converting section 103 inputs transmission data STDAT. The first switch 203 is controlled based on the ON/OFF state of a symbol multiple signal SOGIM for selection of the guard interval section length. The first switch 203 selectively outputs one of an optimal guard interval section length selection signal SOGIS or a parallel output of p symbols showing the guard interval section length. The inverse Fourier transforming section 104 carries out inverse Fourier transform. The guard interval adding section 105 inserts a guard interval section based on a guard interval section length data SIGL.
Also, a receiving apparatus 202 is composed of a guard interval removing section 106, a Fourier transforming section 107, a parallel/serial converting section 108, a demodulating section 109, a second switch 204, a serial/parallel converting section 205, determining sections 206 and 207, and a guard interval length determining section 208. The guard interval removing section 106 removes the guard interval section from a reception signal SRX based on the guard interval section length data SIGL. The Fourier transforming section 107 carries out Fourier transform. The parallel/serial converting section 108 carries out parallel/serial conversion to Fourier transform output signals. The second switch 204 is controlled based on the ON/OFF state of a guard interval section length selection symbol process signal SOGIP, and outputs one of the Fourier transform output signals to the parallel/serial converting section 108 or serial/parallel converting section 205. The demodulating section 109 demodulates reception data from the output of the parallel/serial converting section 108. The determining sections 206 and 207 determine the outputs of the serial/parallel converting section 205. The guard interval length determining section 208 determines the guard interval section length based on determination error signals outputted from the determining sections 206 and 207, and outputs the guard interval section length data SIGL. The operations of the OFDM transmitting and receiving apparatuses shown in FIGS. 2A and 2B are as follows.
In the transmitting apparatus 201, the serial/parallel converting section 103 is controlled by the ON/OFF state of the guard interval section length selection symbol multiple signal SOGIM. When the guard interval section length selection symbol multiple signal SOGIM is in the OFF state, the serial/parallel converting section 103 converts transmission data STDAT into k parallel data signals SPDAT(1) to SPDAT(k)(k is an integer equal to or larger than 2). When the guard interval section length selection symbol multiple signal SOGIM is in the ON state, the serial/parallel converting section 103 converts the transmission data STDAT into (k−1) parallel data signals SPDAT(2) to SPDAT(k). The first switch 203 is controlled in accordance with the ON/OFF state of the guard interval section length selection symbol multiple signal SOGIM. When the guard interval section length selection symbol multiple signal SOGIM is in the OFF state, the first switch 203 selects SPDAT(1) which is one of the k parallel data signals. When the guard interval section length selection symbol multiple signal SOGIM is in the ON state, and the first switch 203 selects the optimal guard interval section length selection signal SOGIS which consists of p symbols showing p (p is an integer equal to or larger than 2) kinds of different guard interval section lengths. Thus, the first switch 203 outputs the selected signal as a first switch output signal SSWO1. The inverse Fourier transforming section 104 carries out inverse Fourier transform to the first switch output signal SSWO1 and the parallel data signal SPDAT(2) to SPDAT(k), and outputs an inverse Fourier transform output signal SIFFTO. The guard interval adding section 105 is controlled based on the guard interval section length data SIGL, copies a part of inverse Fourier transform output signal SIFFTO which is specified by the guard interval section length data SIGL, adds to the inverse Fourier transform output signal SIFFTO as a guard interval section, and outputs as a transmission signal STX.
In the receiving apparatus 202, the guard interval removal section 106 removes the guard interval section from a reception signal SRX based on the guard interval section length data SIGL to be described later, and outputs as a Fourier transform input signal SFFTI. The Fourier transforming section 107 carries out Fourier conversion to the Fourier transform input signal SFFTI, and outputs Fourier transform output signals SFFTO(1) to SFFTO(k). The second switch 204 outputs SFFTO(1) which is one of the k Fourier transform output signals as a second switch first output signal SSWO21, when the guard interval section length selection symbol process signal SOGIP in the ON state. Also, the second switch 204 outputs the same Fourier transform output signal SFFTO(1) as a second switch second output signal SSWO22, when the guard interval section length selection symbol process signal SOGIP is in the OFF state. The serial/parallel converting section 205 carries out serial/parallel conversion to the second switch first output signal SSWO21 to output p determining section input signals SDETI(1) to SDETI(p) The first determining section 206 to the p-th determining section 207 output determination error signals SDERR(1) to SDERR(p) as the determining section input signal SDETI(1) to SDETI(p). The guard interval length determining section 208 determines the guard interval section length and outputs the guard interval section length data SIGL. The guard interval section length is used in the next transmission by the guard interval adding section 105 of the transmitting apparatus 101 based on the determination error signals SDERR(1) to SDERR(p) corresponding to the p kind of guard interval section lengths. The parallel/serial converting section 108 carries out parallel/serial conversion to the (k−1) Fourier transform output signals SFFTO(2) to SFFTO(k), when the guard interval section length selection symbol process signal SOGIP is in the ON state, and carries out the parallel/serial conversion to the (k−1) Fourier transform output signals SFFTO(2) to SFFTO(k) and the second switch second output signal SSWO22 when the guard interval section length selection symbol process signal SOGIP is in the OFF state. Thus, parallel/serial converting section 108 outputs a demodulating section input signal SIDEM. The demodulating section 109 determines the transmission data based on the demodulating section input signal SIDEM and outputs a reception data signal SRDAT.
As such, in the OFDM transmitting and receiving apparatuses shown in FIGS. 2A and 2B, the optimal guard interval section length selection signal include a plurality of symbols showing a plurality of different guard interval section lengths and is supplied to the inverse Fourier transforming section 104, which receives to the respective frequency signals (sub-carriers) in a frequency domain. Therefore, the optimal guard interval section length selection signal is multiplexed to one of the sub-carriers in a time domain signal outputted from inverse Fourier transforming section 104. In this way, in the receiving apparatus 202, the determination error signal is detected to each symbol, and the control of the optimal guard interval section length can be made possible in the anti-multi-path characteristic and the data transmission efficiency.
However, in the above-mentioned conventional OFDM transmitting and receiving apparatuses, the optimal guard interval section length selection signal SOGIS is multiplexed with one sub-carrier to select the optimal guard interval section length. The sub-carrier is used for a purpose other than an original signal transmission. Therefore, the data transmission efficiency is reduced. Also, the conversion ratio in the serial/parallel conversion of the transmission data in the transmitting apparatus and the conversion ratio of the parallel/serial conversion in the reception apparatus are different between a time interval during which the optimal guard interval section length selection signal is multiplexed and a time interval during which the optimal guard interval section length selection signal is not multiplexed based on switching operations by the first and second switches 203 and 204. Therefore, the structure and operation of the transmitting apparatus and reception apparatus becomes complicated.
In conjunction with the above description, an automatic frequency control method is disclosed in Japanese Laid Open Patent application (JP-P-Heisei 9-102774). In this conventional example, an OFDM signal having a guard interval is received, and a frequency conversion is carried out to the OFDM reception signal into a complex modulation signal at a base band by an orthogonal detector using a local oscillation signal. The complex modulated signal is converted into a digital signal. A discrete Fourier transform is carried out to a part of the guard interval section of the digital signal. Also, a discrete Fourier transform is carried out to a part of an effective symbol section apart from the part of the guard interval section from by the effective symbol section. A complex division is carried out to the above discrete Fourier transform results, and the complex division result is phase-converted. The phase conversion result is converted into an analog signal, which is filtered. The local oscillation signal is controlled based on the filtering result.
Also, a digital communication apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2000-22660A). In this conventional example, a frequency converting section inputs an OFDM signal in which a pilot symbol containing a predetermined known pilot signal is inserted and obtains a base band signal using a reproduction carrier. A window function calculating section multiplies the base band signal by a function of window except for a rectangular window. A decoding section converts the output of the window function calculating section into the output of a frequency domain. A known sequence generating section generates a sequence based on the predetermined known sequence and the window function for every carrier frequency. A frequency error estimating section estimates a frequency error of the reproduction carrier used by the frequency converting section based on a correlation of the output from the decoding section and a sequence generated by the known sequence generating section and controls the frequency error of the reproduction carrier.
Also, a multi-carrier reception apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2000-269930A). In this example, a signal of an effective symbol length is extracted from a signal composed of an effective symbol and a guard interval in which a part of the effective symbol is copied and a time window is determined fro discrete Fourier transform. A modulated reception signal in a phase modulation is orthogonally demodulated, and the signal of the effective symbol length is extracted at timing from the demodulated signal. Discrete Fourier transform is carried out to each of the signals of the effective symbol length, and the complex vectors of the different carriers in frequency as result of the discrete Fourier transform are compared. Thus, the effective symbol extracting timing is controlled in feedback such that the variance relating value with the complex vectors of the carriers takes a minimal value.
Also, an OFDM transmitting apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2001-69112A). In this example, data sequences of many partial channels is OFDM modulated in a predetermined frequency band and transmitted, and the data sequence of a desired channel from the data sequences of many partial reception channels can be demodulated. In an OFDM transmission apparatus, a mapping circuit carries out a mapping operation to a plurality of data sequences having a different central frequency and a different bandwidth independently. A multiplexing unit multiplexes in frequency the plurality of mapping signals. A frequency converter 18 converts in frequency the orthogonal modulation signal based on the central frequency of the central frequencies of the plurality of data sequences. In an OFDM reception apparatus, a channel selector sets a frequency of the data sequence to be selected from the received channels. A frequency converter converts in frequency a signal of a frequency obtained by adding a middle frequency to the selected frequency and a received signal. A frequency converter carries out a frequency correction in accordance with a difference between the difference of the central frequency of the center frequencies of the plurality of data sequences and the selected frequency.